06-12-2010, 08:02 AM
1. I think the discussion started from the seismic point of view (read post#1).
2. The non-linear analysis offers you the possibility to view the sequence of formation of the plastic hinges. Plastic design is a different matter. Codes means EC8, our national seismic design code and others. Where have I said that codes forbid plastic analysis? Don't misinterpret what I have written! If plastic design of structures is the topic of the discussion, please state it!
3. About the post #2, Where is it said that it is OK to have plastic hinges in columns? The meaning of what it is written in post #2 is: plastic hinges can form in columns, but in order to have a story mechanism which can lead to collapse, all the columns in a story must have plastic hinges at the top and at the bottom.
4. The codes allow the following situation: the formation of plastic hinges in the top story columns. The reasons are quite obvious.
5. The codes do not require non-linear analysis for current building design. It is rather a designer's choice.
6. The formation of plastic hinges is due to the bending moment. An increased axial force in columns can increase the bending column capacity (up to a certain limit, of course). But if it is assumed that the columns are designed according to modern codes (the axial force is limited) than an increased axial force will be better than a decreased axial force.
I hope that things are clear, now!
Now returning to plastic hinges I will quote a few things from 'Seismic Design of Reinforced Concrete and Masonry Building':
"The primary aim of the capacity design of columns is to eliminate the likelihood of the simultaneous formation of plastic hinges at both ends of all columns of a story". It refers to columns above level 2.
"Moreover, during the inelastic dynamic response of a frame, when frame distortions similar to those of higher mode shapes occur, moments may significantly increase at one or the other end of a column, and hence the formation of plastic hinge at either ends must be expected. Accordingly, relevant codes specify that each end of the such a column be designed and detailed for adequate rotational ductility."
Regarding the use of concrete shear walls I will quote some relevant things from the "Designer's Guide to EN1998-1 and EN 1998-5'
"So, in the horizontal direction of the building that has walls resisting at least 50% of the seismic base shear, Eurocode 8 expects these walls to prevent the occurrence of a soft -story mechanism, and waives the condition of satisfaction of equation D4.23 at the joints of primary seismic columns with beams".Equation D4.23 refers to the capacity design of frames against plastic hinging in columns.
Regardingto to the capacity design of frames against plastic hinging in columns.
"The objective of Eurocode 8 rules for the design of (concrete, steel or composite) moment-resisting frames is to force plastic hinges out of the columns and into the beams, so that a beam-sway mechanism develops and a soft-story is prevented".
Also regarding to primary and secondary seismic elements, from the same source:
"The building structures is taken in design to rely for its earthquake resistance only on its primary seismic elements....The strength and stiffness of secondary elements against lateral loads is to be neglected in the analysis for the seismic action. However, their contribution in resisting other actions (mainly gravity loads) should be fully accounted for".
There is also a section about "Special Requirements for the design of secondary seismic elements" which describes the design of these elements in the same above-mentioned source.
So, I hope I made myself clear now: the objective of the capacity design is to avoid the formation of a soft story mechanism (which means to have simultaneous f plastic hinges at both ends of all columns of a story)!
2. The non-linear analysis offers you the possibility to view the sequence of formation of the plastic hinges. Plastic design is a different matter. Codes means EC8, our national seismic design code and others. Where have I said that codes forbid plastic analysis? Don't misinterpret what I have written! If plastic design of structures is the topic of the discussion, please state it!
3. About the post #2, Where is it said that it is OK to have plastic hinges in columns? The meaning of what it is written in post #2 is: plastic hinges can form in columns, but in order to have a story mechanism which can lead to collapse, all the columns in a story must have plastic hinges at the top and at the bottom.
4. The codes allow the following situation: the formation of plastic hinges in the top story columns. The reasons are quite obvious.
5. The codes do not require non-linear analysis for current building design. It is rather a designer's choice.
6. The formation of plastic hinges is due to the bending moment. An increased axial force in columns can increase the bending column capacity (up to a certain limit, of course). But if it is assumed that the columns are designed according to modern codes (the axial force is limited) than an increased axial force will be better than a decreased axial force.
I hope that things are clear, now!
Now returning to plastic hinges I will quote a few things from 'Seismic Design of Reinforced Concrete and Masonry Building':
"The primary aim of the capacity design of columns is to eliminate the likelihood of the simultaneous formation of plastic hinges at both ends of all columns of a story". It refers to columns above level 2.
"Moreover, during the inelastic dynamic response of a frame, when frame distortions similar to those of higher mode shapes occur, moments may significantly increase at one or the other end of a column, and hence the formation of plastic hinge at either ends must be expected. Accordingly, relevant codes specify that each end of the such a column be designed and detailed for adequate rotational ductility."
Regarding the use of concrete shear walls I will quote some relevant things from the "Designer's Guide to EN1998-1 and EN 1998-5'
"So, in the horizontal direction of the building that has walls resisting at least 50% of the seismic base shear, Eurocode 8 expects these walls to prevent the occurrence of a soft -story mechanism, and waives the condition of satisfaction of equation D4.23 at the joints of primary seismic columns with beams".Equation D4.23 refers to the capacity design of frames against plastic hinging in columns.
Regardingto to the capacity design of frames against plastic hinging in columns.
"The objective of Eurocode 8 rules for the design of (concrete, steel or composite) moment-resisting frames is to force plastic hinges out of the columns and into the beams, so that a beam-sway mechanism develops and a soft-story is prevented".
Also regarding to primary and secondary seismic elements, from the same source:
"The building structures is taken in design to rely for its earthquake resistance only on its primary seismic elements....The strength and stiffness of secondary elements against lateral loads is to be neglected in the analysis for the seismic action. However, their contribution in resisting other actions (mainly gravity loads) should be fully accounted for".
There is also a section about "Special Requirements for the design of secondary seismic elements" which describes the design of these elements in the same above-mentioned source.
So, I hope I made myself clear now: the objective of the capacity design is to avoid the formation of a soft story mechanism (which means to have simultaneous f plastic hinges at both ends of all columns of a story)!